Ordinary 2-D seismic data is acquired along lines (See lines 10 and 11 in FIG. 1) that consist of geophone arrays onshore or hydrophone streamer traverses offshore. Geophones and hydrophones act as sensors to receive energy that is transmitted into the ground and reflected back to the surface from subsurface rock interfaces 12. Energy is usually provided onshore by vibroseis vehicles which transmit pulses by shaking the ground at pre-determined intervals and frequencies on the surface. Offshore, airgun sources are usually used. Subtle changes in the energy returned to surface often reflect variations in the stratigraphic, structural and fluid contents of the reservoirs.
In 3-D seismic the principle is similar, however, lines and arrays are more closely spaced (See FIGS. 1 and 2) to provide more detailed sub-surface coverage. With this high density coverage, extremely large volumes of digital data need to be recorded, stored and processed before final interpretation can be made. Processing requires extensive computer resources and complex software to enhance the signal received from the subsurface and to mute accompanying noise which masks the signal.
Once the data is processed, geophysical staff compile and interpret the 3-D seismic information in the form of a 3-D cube (See FIG. 4) which effectively represents a display of subsurface features. Using the data cube, information can be displayed in various forms. Horizontal time slice maps can be made at selected depths (See FIG. 5). Using a computer workstation an interpreter can slice through the field to investigate reservoir issues at different horizons. Vertical slices or sections can also be made in any direction using seismic or well data. Time maps can be converted to depth to provide a structural interpretation at a specific level.
Three-dimensional (3-D) seismic is being used extensively worldwide to provide a more detailed structural and stratigraphic image of subsurface reservoirs. Acceptance of 3-D seismic has accelerated during the last five years based on a proven track record that continues to grow. The 3-D payout has been measured by increased reserve estimates, cost savings from more accurate positioning of delineation and development wells, improved reservoir characterization leading to better simulation models, and the ability to predict more accurately future opportunities and problems during the production history of a field. More importantly, 3-D seismic has also been used as an exploration tool to reduce drilling risk in structurally complex areas and to predict reservoir quality in undrilled areas.
As good as 3-D seismic surveys and interpreters have become, improvements are needed.
In particular, seismic data has been traditionally acquired and processed for the purpose of imaging seismic reflections. Changes in stratigraphy are often difficult to detect on traditional seismic displays due to the limited amount of information that stratigraphic features present in a cross-section view. Although such views provide an opportunity to see a much larger portion of these features, it is difficult to identify fault surfaces within a 3-D volume where no fault reflections have been recorded. More importantly, seismic data is not known to have been acquired or used for the purpose of imaging seismic discontinuities instead of seismic reflections.